1. Field of the Invention
The invention relates to a wet clutch plate having lubrication grooves, a clutch unit including the clutch plate, and a driving force transmission system including the clutch plate.
2. Description of Related Art
A friction clutch that performs power transmission by frictionally engaging drive-side clutch plates and driven-side clutch plates with each other has been conventionally known. Further, an electromagnetic friction clutch that includes clutch plates each having a plurality of windows that are arc-shaped through-holes has been known. The windows are required to form magnetic circuits. Such an electromagnetic friction clutch is described in each of Japanese Patent Application Publication No. 11-294488 (JP 11-294488 A), Japanese Patent Application Publication No. 2002-213485 (JP 2002-213485 A) and Japanese Patent Application Publication No. 2006-029446 (JP 2006-029446 A). The electromagnetic clutch is used in, for example, an electronically-controlled 4WD coupling (ITCC (registered trademark)).
Lubricating oil is present between the clutch plates. Further, lubrication grooves are formed in axial end faces (sliding surfaces) of the clutch plates. When the clutch plates are engaged with each other, the lubricating oil is retained in the lubrication grooves, and the lubricating oil escapes from between the clutch plates through the lubrication grooves.
In the wet clutch mechanism in which the lubricating oil is present between the clutch plates as described above, the torque at a low temperature (for example, below or equal to 0° C.) increases as the viscosity of the lubricating oil increases. If the rate of change of a torque (torque change rate) at a low temperature with respect to a torque at a high temperature becomes higher, it becomes difficult to improve, for example, the operational feeling. Therefore, reduction in torque change rate has been demanded.